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The density p of two silicon crystals has been determined by precision hydrostatic weighing, based on very accurate solid density standards (SDS's). Densities of additional crystals were gained from density differences measured by the temperature-of-f1oatation method. The molar mass M of the crystals was determined by comparing the isotopic abundances against the known abundances of the National Bureau of Standards (NBS) reference material SRM 990 using activation analysis and mass spectroscopy. Combining the measured values for M and p leads to M / p = (12.058 822 ± 0.000 013) cnr' / mol. With the known volume of the unit cell the following value for the Avogadro constant was calculated: N A = (6.022 137 ± 0.000 007) x 10 23 mol-I.

The paper summarizes the activities of several national metrological institutes in replacing the kilogram artefact by the mass of a certain number of silicon atoms. This atomic path requires a very precise determination of the Avogadro constant NA with a relative uncertainty of better than 5 × 10-8. At present, the most limiting factors are the variability, from sample to sample, of the isotopic abundances and the content of impurities and vacancies, and difficulty in the realization of accurate density standards. Recently, several improvements in molar mass and density determinations have been made, so a further reduction of the level of relative uncertainty towards 10-7 may be expected in the near future. Beyond that, new experimental methods will be required.

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